1. Field of the Invention
The present invention relates to an emphasis apparatus for use in a VTR for HDTV or the like.
2. Description of the Prior Art
For example, a conventional VTR is provided with a preemphasis circuit disposed on the record system, for emphasizing the high band of a recorded signal; a deemphasis circuit disposed on the playback system, for emphasizing the high band of the playbacked signal; and a deemphasis circuit disposed on the playback system, for deemphasizing the playbacked signal so as to prevent S/N ratio from decreasing in record and playback modes.
A nonlinear emphasis circuit for increasing the emphasizing amount proportional to the frequency of a signal or disproportional to the amplitude of a signal has been practically used. FIGS. 1, 2, and 3 show examples of conventional emphasis circuits. In FIG. 1, reference numeral 10A is a nonlinear preemphasis circuit disposed in a record system. In the circuit, a recorded signal SS such as a brightness signal is supplied from an input terminal 11 to an adder 12. In addition, the received signal SS is also supplied to a high-pass filter 13. The high-pass filter 13 extracts a high band component from the received signal SS. The high band component is supplied to the adder 12 through a nonlinear circuit, for example, a limiter 14, and a coefficient generating circuit 15. Thus, the adder 12 outputs a signal SP whose high band is emphasized or a preemphasized signal.
In this circuit, the limiter 14 limits the emphasizing amount of the high band component to, for example, at most 10 dB. The coefficient generating circuit 15 weights the high band component which is added to the main signal SS which is supplied to the adder 12. The preemphasized signal SP supplied from the adder 12 is supplied to a record and playback system 100 and then recorded or playbacked.
Reference numeral 20A is a nonlinear deemphasis circuit. The signal SP which is playbacked in the record and playback system 100 is supplied to a subtracter 22. An output signal of the subtracter 22 is obtained from an output terminal 21. This signal is also fed back to the subtracter 22 through a signal line which comprises a high-pass filter 23, a limiter 24, and a coefficient generating circuit 25. The circuits 23 to 25 have the same characteristics as the circuits 13 to 15 of the preemphasis circuit 10A, respectively. Thus, since the deemphasis circuit 20A has the reverse (compensating) characteristics of the preemphasis circuit 10A, the signal SS with a flat frequency characteristic is output from the output terminal 21.
Since the above mentioned deemphasis circuit 20A is constructed as a feed-back type, the reverse characteristic (reverse transfer function) in the high band cannot be correctly accomplished due to phase rotation therein.
To solve such a problem, a deemphasis circuit 20B in a feed-forword construction as shown in FIG. 2 has been proposed. In this deemphasis circuit 20B, since a playbacked signal SP passes through a signal line which comprises a high-pass filter 23, a limiter 24, and a coefficient generating circuit 25, the high band component is extracted. The extracted high band component is supplied to a subtracter 22. The subtracter 22 subtracts the high band component from the playbacked signal SP. Thereby, an original signal SS is obtained.
The deemphasis circuit 20B of the feed-forward type can more accurately accomplish the reverse characteristic than the deemphasis circuit 20A shown in FIG. 1. However, in the deemphasis circuit 20B, when the amplitude of the signal exceeds the limiter level of the limiter 24, the distortion of the signal becomes large.
A deemphasis circuit 20C shown in FIG. 3 is constructed so that such distortion is decreased. In this circuit, a signal line of high band component is provided with a low-pass filter 26. In addition, a distortion component .DELTA. is formed by a high-pass filter 33, a limiter 34, a low-pass filter 36, and a coefficient generating circuit 35. A subtracter 27 subtracts the distortion component .DELTA. from the high band component. Thereby, a high band component free of the distortion component .DELTA. is obtained. This high band component is supplied to an subtracter 22 and thereby the original signal SS is obtained from a terminal 21. However, this deemphasis circuit 20C shown in FIG. 3 has three signal routes. Thus, the circuit construction is complicated and the adjustment thereof becomes difficult.
As described above, in the deemphasis circuit 20A as shown in FIG. 1, it is difficult to accomplish the reverse characteristic in the high band. In addition, in the deemphasis circuit 20B as shown in FIG. 2, when the amplitude of a playbacked signal is large, a distortion takes place. Moreover, in the deemphasis circuit 20C as shown in FIG. 3, the construction is complicated and the adjustment thereof becomes difficult.